What others have said is true, and also note that Amazon bought Twitch 2 years ago, so I'm sure Amazon can run their own product announcements through Twitch if they want :)
My guess that was a sponsored deal or something. But as my edit from before it seems that re:invent is being streamed on Twitch anyway so guessing its all above board (and as others have said Amazon owns Twitch).
I'm guessing it is covered under the Twitch Creative Conduct, since there is an entire Creative category now that is getting more popular which involves people painting, cosplay, digital art, etc.